1. Field of the Invention
The present invention relates to a network relay device that carries out data transmission to and from an opposite device, so as to relay data in a network.
2. Description of the Related Art
A switch as one of network relay devices may be designed to have the function of link aggregation, which connects with another switch by multiple links (multiple physical lines) and aggregates the multiple links as one logic link.
One known example of the link aggregation is disclosed in Japanese Patent Laid-Open Gazette No. 2002-26909.
The bandwidth of the logic link is the total of the individual bandwidths of the aggregated multiple links. The link aggregation thus advantageously expands the bandwidth. Even in the case of the occurrence of a trouble or failure in some of the aggregated multiple links, the link aggregation adopts a degenerate mode to continue communication via the remaining normal links and accordingly assures the redundancy.
This prior art technique, however, has the following problems with regard to the redundancy of the link aggregation.
In the case of the occurrence of a trouble in some of the aggregated multiple links, the degenerate mode establishes communication by only normal links and thus undesirably reduces the total bandwidth by the bandwidth of the link with the trouble. This prior art technique continues communication with the undesirable reduced bandwidth, while assuring the redundancy.
Another possible measure to assure the redundancy in the case of the occurrence of a trouble changes the communication path of the network to bypass the link with the trouble.
Application of this measure to the technique of link aggregation establishes two or more link aggregations in one switch and specifies one of the multiple link aggregations as active to establish communication via aggregated multiple links of the active link aggregation. In response to the occurrence of a trouble or failure in some of the aggregated multiple links in the currently active link aggregation, the method of this measure stops the currently active link aggregation and makes a standby link aggregation active.
In the case where a trouble or failure arises in some of the aggregated multiple links in the currently active link aggregation, restraint of the degenerate mode and stop of data transmission via the aggregated links are required to effect a switchover to the standby link aggregation.
This problems is not restricted to the switches as one of the network relay devices but is also found in other network relay devices having the function of link aggregation, for example, routers.